We came to Don't Leave the Path with Ungoliant's Spawn already engaged and a couple of hit points left. The Quest condition is 'players must find and defeat Ungoliant's Spawn to win this game.' So, we ruled that she was 'found' and we won the game easily next round.
It was very fortunate, as presumably, had we defeated US the round before, we would have had to get her from the deck and fight her all over again. I'm wondering if this is the correct interpretation.